I calculated that that my cellphone battery (3.7V, 1.23Ah) can store 16.4kJ of energy and a battery for an RC car (7.2V, 3Ah) can store 77.7kJ, which could recharge the cellphone a few times.So I was wondering if I could make a mobile USB charger of a Racing Pack and a 5V voltage regulator.

My questions are:Does this work at all?Can I damage something (for example with a too high voltage)?What happens if the racing pack is discharged? Can I damage the phone / usb device with a too low voltage?How can I determine when racing pack is discharged?

Please be sure not to over-discharge your battery pack, or you will hurt your batteries. You could make an auto-switch off circuit with some resistors and a transistor that turns off the load when the voltage gets too low.

Looks to me like a problem would be the dropout voltage of 2V. I read this to mean the 7805 will stop outputting when the input voltage dips below 7 volts. That discharging RC battery wouldn't last long like that, would it?